Studies have been devoted to determining how adenylate cyclase activity might be regulated in normal fibroblasts growing in culture. These findings will be of use in trying to elucidate the reasons for altered cyclase activity following viral transformation and during various stages of cell growth. The effects of serum on adenylate cyclase activity is of primary concern since the addition of serum to intact cells decreases intracellular cyclic AMP levels. The addition of serum in vitro selectively inhibits the GTP and hormone-stimulated cyclase activities. Organic solvent extraction of serum resulted in the isolation of a free fatty acid fraction which was very inhibitory toward adenylate cyclase when the free fatty acid to albumin ratio exceeded 5. Unsaturated fatty acids inhibit basal, hormonal, and fluoride activities, while the saturated fatty acids and methyl esters of various chain lengths were without effect. A high molecular-weight factor has been partially purified from serum which demonstrates the selective inhibition of the GTP function; it has little effect on basal or fluoride-stimulated activity. Work is in progress to further purify and characterize this serum factor.